Laser produced plasma euv light source

ABSTRACT

An EUV light source is disclosed that may include a laser source, e.g. CO 2  laser, a plasma chamber, and a beam delivery system for passing a laser beam from the laser source into the plasma chamber. Embodiments are disclosed which may include one or more of the following; a bypass line may be provided to establish fluid communication between the plasma chamber and the auxiliary chamber, a focusing optic, e.g. mirror, for focusing the laser beam to a focal spot in the plasma chamber, a steering optic for steering the laser beam focal spot in the plasma chamber, and an optical arrangement for adjusting focal power.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent application Ser. No. 11/358,992, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE, filed on Feb. 21, 2006, Attorney Docket No. 2005-0081-01, which is a continuation-in-part application of U.S. patent application Ser. No. 11/174,442, entitled SYSTEMS AND METHODS FOR REDUCING THE INFLUENCE OF PLASMA-GENERATED DEBRIS ON THE INTERNAL COMPONENTS OF AN EUV LIGHT SOURCE, filed on Jun. 29, 2005, Attorney Docket No. 2004-0110-01 which is a continuation-in-part application of U.S. patent application Ser. No. 10/979,945, entitled LPP EUV LIGHT SOURCE, filed on Nov. 1, 2004, Attorney Docket No. 2004-0088-01, the disclosures of each of which are hereby incorporated by reference herein.

U.S. patent application Ser. No. 11/358,992 is also a continuation-in-part application of U.S. patent application Ser. No. 11/067,099, entitled SYSTEMS FOR PROTECTING COMPONENTS OF AN EUV LIGHT SOURCE FROM PLASMA-GENERATED DEBRIS, filed on Feb. 25, 2005, Attorney Docket No. 2004-0117-01 the disclosure of which is hereby incorporated by reference herein.

The present invention is also related to U.S. patent application Ser. No. 10/900,839, entitled EUV LIGHT SOURCE, filed on Jul. 27, 2004, Attorney Docket No. 2004-0044-01, U.S. patent application Ser. No. 10/803,526, entitled HIGH REPETITION RATE LPP EUV LIGHT SOURCE, filed on Mar. 17, 2004, Attorney Docket No. 2003-0125-01, and U.S. patent application Ser. No. 10/798,740, entitled COLLECTOR FOR EUV LIGHT, filed on Mar. 10, 2004, Attorney Docket No. 2003-0083-01, the disclosures of each of which are hereby incorporated by reference herein.

The present application is also related to co-pending U.S. non-provisional patent application entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE filed concurrently herewith, attorney docket number 2005-0085-01, the entire contents of which are hereby incorporated by reference herein.

The present application is also related to co-pending U.S. nonprovisional patent application entitled SOURCE MATERIAL DISPENSER FOR EUV LIGHT SOURCE filed concurrently herewith, attorney docket number 2005-0102-01, the entire contents of which are hereby incorporated by reference herein.

The present application is also related to co-pending U.S. provisional patent application entitled EXTREME ULTRAVIOLET LIGHT SOURCE filed concurrently herewith, attorney docket number 2006-0010-01, the entire contents of which are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to extreme ultraviolet (“EUV”) light sources which provide EUV light from a plasma that is created from a source material and collected and directed to a focus for utilization outside of the EUV light source chamber, e.g., for semiconductor integrated circuit manufacturing photolithography e.g., at wavelengths of around 50 nm and below.

BACKGROUND OF THE INVENTION

Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.

Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam.

For this process, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include heat, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of source material that is not fully ionized in the plasma formation process.

These plasma formation by-products can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, collector mirrors including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and/or grazing incidence, the surfaces of metrology detectors, windows used to image the plasma formation process, and the laser input window. The heat, high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them. For some source materials, e.g. tin, it may be desirable to introduce an etchant, e.g. HBr into the plasma chamber to etch debris that deposits on the optical elements. It is further contemplated that the affected surfaces of the elements may be heated to increase the reaction rate of the etchant.

In addition, some optical elements, e.g., the laser input window, form a part of the vacuum chamber, and heretofore, have typically been placed under a considerable stress due to a pressure differential between the relatively high vacuum in the plasma chamber and the pressure, e.g. atmospheric pressure, outside the plasma chamber. For these elements, deposits and heat can combine to fracture (i.e., crack) the element resulting in a loss of vacuum and requiring a costly repair. To accommodate this stress and prevent fracture, laser input windows have generally been rather thick, and, as a consequence, are subject to thermal lensing. This thermal lensing, in turn, can reduce the ability to properly steer and focus a laser beam to a desired location within the plasma chamber. For example, for use in some LPP EUV light sources, it is contemplated that a laser beam be focused to a spot diameter of about 300 μm or less.

In addition to reducing problems associated with thermal lensing, a laser beam delivery system for an EUV light source may have components that are exposed to the plasma chamber environment. These components may include the laser input window and in some cases focusing and/or steering optics. For these components, it may be desirable to use materials that are compatible with the etchant and heat used in debris mitigation.

With the above in mind, Applicants disclose systems and methods for effectively delivering and focusing a laser beam to a selected location in an EUV light source.

SUMMARY OF THE INVENTION

An EUV light source is disclosed that may include a laser source, e.g. CO₂ laser, a plasma chamber, and a beam delivery system having at least one auxiliary chamber. The auxiliary chamber may have an input window for passing a laser beam from the laser source into the auxiliary chamber and an exit window for passing the laser beam into the plasma chamber.

In one embodiment, a bypass line may be provided to establish fluid communication between the plasma chamber and the auxiliary chamber. A valve may be provided to close the bypass line. In a particular embodiment, a focusing optic, e.g. mirror, may be disposed in the auxiliary chamber for focusing the laser beam to a focal spot in the plasma chamber. In another embodiment, the focusing optic, e.g. mirror, may be disposed in the plasma chamber.

In another aspect, a focusing optic that is moveable to selectively move the location of the focal spot may be provided. In one embodiment, a target delivery system, e.g. droplet generator, may be provided for delivering a plasma source material to a plasma formation site in the plasma chamber together with a target position detector for generating a signal indicative of target position. With this arrangement, the focusing optic may be moveable in response to the signal from the target position detector to locate the focal spot along a target trajectory.

In still another aspect of an embodiment, a steering optic, e.g. flat mirror mounted on a tip-tilt actuator, may be provided for steering the laser beam focal spot in the plasma chamber. In a particular embodiment, the steering optic and focusing optic may be moveable together in a first direction to move the focal spot along a path parallel to the first direction. Like the focusing optic, the steering optic may be moveable in response to a signal from as target position detector to locate the focal spot along a target trajectory. An optical assembly, e.g. z-fold telescope, may be provided to adjust focal power.

For one or more of the above described embodiments, a plasma may be created in the plasma chamber comprising a plasma formation material, e.g. Sn, and an etchant for the plasma formation material, e.g. HBr, HI, Br₂, Cl₂, HCl, H₂ or combinations thereof, may be introduced into the plasma chamber. In a particular embodiment, a heating subsystem may be provided to heat deposited plasma formation material on the exit window to a temperature greater than 150° C. to increase a rate of a chemical reaction between deposited plasma formation material and the etchant.

In one aspect, an EUV light source may include a laser source having a discharge chamber operating at a discharge pressure, e.g. CO₂ laser, a plasma chamber, and a beam delivery system having at least one auxiliary chamber. The auxiliary chamber may have an input window for passing a laser beam from the laser source into the auxiliary chamber and an exit window for passing the laser beam into the plasma chamber. For this aspect, the light source may further include a bypass line establishing fluid communication between the discharge chamber and the auxiliary chamber. A valve operable to close the bypass line may be provided. In one particular embodiment, the auxiliary chamber may include a first compartment, a second compartment, and a sealing assembly for reconfiguring the auxiliary chamber between a first configuration wherein the first compartment is in fluid communication with the second compartment and a second configuration wherein the first compartment is sealed from the second compartment. For example, a window may be disposed in the auxiliary chamber that is moveable between a first position wherein the first compartment is in fluid communication with the second compartment and a second position wherein the first compartment is sealed from the second compartment.

In another aspect, a light source that produces EUV by plasma formation and generates debris may include a plasma chamber, a CO₂ laser source producing a laser beam having a wavelength, λ, e.g. about 10.6 μm, and a laser input window for the plasma chamber. The light source may introduce an etchant, e.g. HBr, HI, Br₂, Cl₂, HCl, H₂ or a combination thereof, for the plasma formation material, e.g. Sn, into the plasma chamber and a subsystem may be provided for heating deposited plasma formation material on the window to an elevated temperature, t, e.g., a temperature greater than about 150° C., to increase a rate of a chemical reaction between deposited plasma formation material and the etchant. For this aspect, the window provided may be substantially transparent to light at the wavelength, λ, and the window may have a surface that is chemically stable when exposed to the etchant at the temperature, t. In one embodiment, the window may be made of a material such as KBr, CsBr or CsI. In another embodiment, the window may be made by coating a material with KBr, CsBr or CsI. In a particular embodiment, a coating system to coat the window with a material selected from the group of materials consisting of KBr, CsBr and CsI after the window may be installed in the plasma chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an overall broad conception for a laser-produced plasma EUV light source according to an aspect of the present invention;

FIG. 2 shows a schematic, sectional view of an aspect of an embodiment wherein a beam delivery system is provided to deliver a laser beam into an EUV plasma chamber;

FIG. 3 shows a schematic, sectional view of another aspect of an embodiment wherein a chamber of a beam delivery system is maintained at a same pressure as a discharge chamber of an LPP laser source;

FIG. 4 shows a schematic, sectional view of an aspect of an embodiment wherein a beam delivery system is provided to deliver a laser beam into an EUV plasma chamber, the beam delivery system including a focusing optic, a steering optic, and an optical assembly to adjust focal power;

FIG. 5 shows a schematic, sectional view of another aspect of an embodiment wherein a beam delivery system is provided to deliver a laser beam into an EUV plasma chamber, the beam delivery system including a focusing optic, a steering optic, and an optical assembly to adjust focal power;

FIG. 6 shows a schematic, perspective view of portions of an EUV light source embodiment showing a fluorescent converter for measuring EUV light output;

FIG. 7 shows a fluorescent converter image and corresponding intensity plot for an EUV output pulse generated by a laser—droplet interaction in which the laser is off-center relative to the droplet; and

FIG. 8 shows a fluorescent converter image and corresponding intensity plot for an EUV output pulse generated by a laser—droplet interaction in which the laser is centered relative to the droplet.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With initial reference to FIG. 1 there is shown a schematic view of an exemplary EUV light source, e.g., a laser produced plasma EUV light source 20 according to an aspect of the present invention. As shown, the LPP light source 20 may contain a pulsed or continuous laser system 22, e.g., a pulsed gas discharge CO₂, excimer or molecular fluorine laser operating at high power and high pulse repetition rate. Depending on the application, other types of lasers may also be suitable. For example, a solid state laser, a MOPA configured excimer laser system, e.g., as shown in U.S. Pat. Nos. 6,625,191, 6,549,551, and 6,567,450, an excimer laser having a single chamber, an excimer laser having more than two chambers, e.g., an oscillator chamber and two amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator/power oscillator (MOPO) arrangement, a power oscillator/power amplifier (POPA) arrangement, or a solid state laser that seeds one or more CO₂, excimer or molecular fluorine amplifier or oscillator chambers, may be suitable. Other designs are possible.

The light source 20 may also include a target delivery system 24, e.g., delivering targets, e.g. targets of a source material including tin, lithium, xenon or combinations thereof, in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The targets may be delivered by the target delivery system 24, e.g., into the interior of a chamber 26 to an irradiation site 28 where the target will be irradiated and produce a plasma.

A beam delivery system 50 having an auxiliary chamber 52 may be provided to deliver a laser beam from the laser source 22, e.g. a beam of laser pulses, along a laser optical axis into the plasma chamber 26 to the irradiation site 28. At the irradiation site, the laser, suitably focused, may be used to create a plasma having certain characteristics which depend on the source material of the target. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma. As shown, the beam deliver system 50 may have an input window 54 for passing a laser beam from the laser source 22 into the auxiliary chamber 52 and an exit window 56 for passing the laser beam into the plasma chamber 26.

Continuing with FIG. 1, the light source 20 may also include a collector 30, e.g., a reflector, e.g., in the form of a truncated ellipse, with an aperture to allow the laser light to pass through and reach the irradiation site 28. The collector 30 may be, e.g., an elliptical mirror that has a first focus at the irradiation site 28 and a second focus at a so-called intermediate point 40 (also called the intermediate focus 40) where the EUV light may be output from the light source 20 and input to, e.g., an integrated circuit lithography tool (not shown).

The light source 20 may also include an EUV light source controller system 60, which may also include, e.g., a target position detection feedback system 62 and a laser firing control system 65, along with, e.g., a laser beam positioning system controller 66. The light source 20 may also include a target position detection system which may include one or more droplet imagers 70 that provide an output indicative of the position of a target droplet, e.g., relative to the irradiation site 28 and provide this output to the target position detection feedback system 62, which can, e.g., compute a target position and trajectory, from which a target error can be computed, if not on a droplet by droplet basis then on average. The target error may then be provided as an input to the light source controller 60, which can, e.g., provide a laser position, direction and timing correction signal, e.g., to the laser beam positioning controller 66 that the laser beam positioning system can use, e.g., to control the laser timing circuit and/or to control a laser beam position and shaping system 68, e.g., to change the location and/or focal power of the laser beam focal spot within the chamber 26.

As shown in FIG. 1, the light source 20 may include a target delivery control system 90, operable in response to a signal from the system controller 60 to e.g., modify the release point of the target droplets as released by the target delivery mechanism 92 to correct for errors in the target droplets arriving at the desired irradiation site 28. An EUV light source detector 100 may be provided to measure one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths and/or average power, and generate a feedback signal for use by the system controller 60 that can be, e.g., indicative of the errors in such things as the timing and focus of the laser pulses to properly intercept the target droplets in the right place and time for effective and efficient EUV light production. Although the detector 100 is shown positioned to receive light directly from the irradiation site 28, it is to be appreciated the detector could also be positioned to sample light at or downstream of the intermediate focus 40.

For the light source 20 shown in FIG. 1, irradiation of the source material targets may generate a plasma, and in some cases, debris may be generated at the irradiation site 28 which may contaminate the surfaces of optical elements including but not limited to the laser input window 56. As shown, a source 102 of a gaseous etchant capable of reaction with the source material may be introduced into the chamber 26 to clean contaminants surfaces of optical elements. For example, an HBr concentration of a few Torr can be used. In one application, the source material may include Sn and the etchant may be HBr, Br₂, Cl₂, HCl, H₂, HCF₃ or a combination thereof.

Continuing with FIG. 1, the EUV light source 20 may include one or more heater(s) 104 to initiate and/or increase a rate of a chemical reaction between deposited source material and the etchant on a surface of an optical element. For example, for a Sn source material and HBr etchant, the heater 104 may heat the contaminated surface of an optical element, e.g. laser input window 56, to a temperature in the range of 150 to 400° C., and for some applications greater than 400° C. For a plasma source material which comprises Li, the heater 104 may be designed to heat the surface or one or more optical elements to a temperature in the range of about 400 to 550° C. to vaporize Li from the surface (i.e. without necessarily using an etchant). Types of heaters which may be suitable include, but are not necessarily limited to radiative heaters, microwave heaters, RF heaters, ohmic heaters and combinations thereof. The heater(s) may be directed to a specific optical element surface, and thus be directional, or may be nondirectional and heat the entire chamber or substantial portions thereof.

FIG. 2 shows an aspect of an embodiment wherein a beam delivery system 250 is provided to deliver a laser beam into an EUV plasma chamber 226. As shown, the beam delivery system 250 may establish an auxiliary chamber 252 which has an input window 254, which may be for example a gas discharge laser output window, for passing a laser beam 220 from the laser source 222 into the auxiliary chamber 252. The system 250 may also include an exit window 256 for passing the laser beam from the auxiliary chamber 226 into the plasma chamber 226.

For the embodiment shown in FIG. 2, a bypass line 200 establishes fluid communication between the plasma chamber 226 and the auxiliary chamber 252. With this arrangement, a substantially same pressure (e.g. a pressure differential of less than about 0.1 atm) may be provided in both chambers 226, 252. With this low pressure differential across the window 256, a relatively thin window may be employed, which in turn, may result in reduced thermal lensing as compared to a thicker window. FIG. 2 also shows that a valve 224 (controllable manually or automatically) may be provided to close the bypass line 200. With this arrangement, the valve 224 can be closed to allow one of the chambers 226, 252 to be accessed or otherwise operated on while maintaining the vacuum state of the other chamber 226, 252. Continuing with FIG. 2, it can be seen that a focusing optic, which for the embodiment shown is a pair of mirrors 228 a, 228 b, may be disposed in the auxiliary chamber 252 for focusing the laser beam 220 to a focal spot 232 in the plasma chamber 226.

FIG. 3 illustrates another aspect of an embodiment wherein a chamber 352 of a beam delivery system 350 may be maintained at a substantially same pressure (e.g. a pressure differential of less than about 0.1 atm) as a discharge chamber 300 of a Laser-Produced-Plasma (LPP) laser source 322. For example, a CO₂ discharge laser may have a discharge chamber pressure of about 0.1 to 0.3 atm, and thus, for the embodiment shown in FIG. 3, the beam delivery system chamber 352 may be maintained at or near this pressure during operation of the EUV light source. In greater structural detail, the auxiliary chamber 352 may have an input window 354 for passing a laser beam (not shown) from the laser source 322 into the auxiliary chamber 352 and a bypass line 302 establishing fluid communication between the discharge chamber 300 and the auxiliary chamber 352. FIG. 3 also shows that a valve 304 (controllable manually or automatically) may be provided to close the bypass line 302. With this arrangement, the valve 304 can be closed to allow one of the chambers 300, 352 to be accessed or otherwise operated on while maintaining the vacuum state of the other chamber 300, 352.

FIG. 3 also illustrates that the auxiliary chamber 352 may include a first compartment 306 a, a second compartment 306 b, and a sealing assembly, which for the embodiment shown is a moveable window 308, for reconfiguring the auxiliary chamber 352 between a first configuration wherein the first compartment 306 a is in fluid communication with the second compartment 306 b and a second configuration wherein the first compartment 306 a is sealed from the second compartment 306 b. More specifically, as shown, window 308 may be disposed in the auxiliary chamber 308 and hinged for movement from a first window position (illustrated by the dash lines) wherein the first compartment 306 a is in fluid communication with the second compartment 306 b and a second window position (illustrated by the solid lines) wherein the first compartment 306 a is sealed from the second compartment 306 b. With this arrangement, the relatively thin input window 354 may be protected against pressure differentials, which may fracture the window 354. In one setup, pressure measurements in the chamber compartments 306 a, 306 b may be taken and used to automatically control the position of the moveable window 308 (e.g. window 308 is only moveable into the first window position when a substantially same pressure is present in the compartments 306 a, 306 b.

FIG. 4 shows an aspect of an embodiment wherein a beam delivery system is provided to focus and steer a laser beam 400 to a focal spot at a desired location in an EUV plasma chamber. As shown there, the light source 420 may include a conical shroud 402 (i.e. a so-called gas cone) that is positioned in the chamber 426 which separates the chamber 426 into two compartments 426 a, 426 b while maintaining fluid communication between the compartments 426 a,b. In one aspect, the shroud 402 may limit (e.g. block) plasma-generated debris from reaching compartment 426 b. In some implementations, a gaseous etchant, e.g. HBr, etc., may be introduced into the shroud 402 and flow out of the shroud 402 through the open end 427. Thus, etchant flow may be directed toward the irradiation site, as shown. In one implementation, a gas pressure, e.g. from a gaseous etchant, in compartment 426 b may be maintained slightly higher than the gas pressure in compartment 426 a to maintain a flow of gas toward the irradiation site.

Continuing with FIG. 4, laser input window 450 seals chamber 426 while allowing laser beam 400 to enter. The hollow shroud 402 allows the laser beam 400 to pass through the shroud 402 and reach the irradiation site. FIG. 4 also shows that a focus and steering assembly 451 is disposed in the compartment 426 a of the chamber 426 and includes a focusing optic which may include one or more mirrors, prisms, lenses, etc arranged to focus a light beam to a focal spot. For the embodiment shown, a mirror 452, which may be an off-axis parabolic mirror, is used to focus the beam 400 to a focal spot at the desired irradiation site 428. The focus and steering assembly 451 also includes a steering optic, which may include one or more mirrors, prisms, lenses, etc. arranged to steer the focal spot established by the focusing optic to a desired location in the plasma chamber 426. For the embodiment shown, the steering optic includes a flat mirror 454 mounted on a tip-tilt actuator 456 which may move the mirror 454 independently in two dimensions. In addition to the two-dimensional movement of the focal spot afforded by the tip-tilt actuator 456, movement of the focal spot in the direction of arrow 458 may be obtained by the selected movement of the focus and steering assembly 451 parallel to the direction indicated by arrow 458. Thus, the steering optic and focusing optic may be moveable together to move the focal spot along a path parallel to arrow 458.

FIG. 4 also shows that the light source 420 may include an optical assembly having one or more mirrors, prisms, lenses, etc. arranged to adjust focal power. For the particular embodiment shown in FIG. 4, the assembly includes two spherical mirrors 460 a,b that are disposed outside the chamber 426 in an optical arrangement commonly known as a z-fold telescope. As FIG. 4 illustrates, one or both of the mirrors 460 a,b can be selectively moved parallel to respective direction arrows 462 a,b to adjust focal power.

FIG. 4 also shows that two turning mirrors 464 a,b are provided to direct the light beam 400 from the z-fold telescope arrangement to the focusing mirror 452. With the arrangement shown in FIG. 4, the turning mirror 464 a is disposed outside the chamber 426 while the turning mirror 464 b is disposed inside the chamber 426. Notwithstanding the use of the shroud 402 to limit the presence of plasma-generated debris in compartment 426 b of chamber 426, it is contemplated that debris mitigation techniques may be employed in compartment 426 b to maintain the surfaces of optical components in compartment 426 b free of debris. For example, the use of an etchant, e.g. HBr with optical components 454, 452, 464 b kept at elevated temperatures may be used. Another feature of the arrangement shown in FIG. 4 is the placement of the laser input window 450. As shown, the window is position such that it is not in direct “line of sight” with the irradiation site where plasma generated debris originates. In addition, the window 450 is positioned at a relatively large distance from the irradiation site, and, in some cases may exposed to a lower temperature (as compared to a closer window) and less debris.

FIG. 5 shows an embodiment of a light source 520 that is similar to the light source 420 having a multi-compartment plasma chamber 526, conical shroud 502, a laser input window 550, a focus and steering assembly 551 disposed in the compartment 526 a of the chamber 526, an optical assembly, e.g. z-fold telescope, having two spherical mirrors 560 a,b that are disposed outside the chamber 526 to adjust focal power, and two turning mirrors 564 a,b are provided to direct the light beam 500 from the z-fold telescope arrangement to the focusing mirror 552. With the arrangement shown in FIG. 5, both turning mirrors 564 a,b are disposed outside the chamber 526, and are thus not exposed to plasma-generated debris and/or elevated temperatures/etchant.

As indicated above, the optical components in the focus and steering assembly 451/551 and mirrors 460 a,b/560 a,b of the focal power adjustment assembly may be moveable to adjust the position and focal power of the focal spot in the chamber 426/526. Adjustments to the focus and steering assembly and/or the focal power adjustment assembly may be made during light source setup and maintained at constant settings during operational use, or, adjustments to the focus and steering assembly and/or the focal power adjustment assembly may be made during light source operational use (i.e. during utilization of EUV light by a downstream apparatus, e.g. lithography scanner), for example, on a pulse by pulse basis and/or after a so-called “burst” of pulses, when a pulsed laser source 22 (see FIG. 1) is used. Adjustments to the focus and steering assembly and/or the focal power adjustment assembly may be made manually (e.g. during setup) or by a controller system 60 (see FIG. 1). For this purpose, inputs to the controller system 60 may include, but are not limited to, signals from the EUV light source detector 100 and/or the target position detection feedback system 62 (see FIG. 1).

FIG. 6 shows portions of an EUV light source embodiment having a collector 30′, e.g., multilayer Mo/Si reflector, e.g., in the form of a truncated ellipse, with an aperture 700 to allow the laser light 702 to pass through and reach the irradiation site 28′ and then pass to a beam stop 704. For the embodiment shown, the elliptical collector 30′ is positioned to have a first focus at an irradiation site 28′ and a second focus at a so-called intermediate point 40′ (also called the intermediate focus 40′) where the EUV light may be output from the light source and input to, e.g., an integrated circuit lithography tool (not shown). As shown, source material from a target delivery mechanism 92′, e.g. source material droplet generator, may reach the irradiation site 28′ for interaction with the laser beam to produce an EUV emission.

As further shown, a detector 706, which may be a fluorescent converter such as a Zr coated Ce:YAG fluorescent converter may be positioned at or near the intermediate focus 40′, e.g. downstream of the intermediate focus 40′, as shown, to measuring in-band EUV light output intensity and producing an output signal indicative thereof. For the embodiment shown, the detector 706 may be periodically interposed within the EUV output beam, e.g. temporarily taking the light source “off line” or may sample a portion of the EUV light output, for example, using a pick-off type beam splitter (not shown). Although a fluorescent converter is shown, it is to be appreciated that other types of detectors known to those skilled in the art may be used to measure EUV output intensity, in-band or otherwise, as described herein.

FIG. 7 shows an example of a fluorescent converter image (note: light areas indicate in-band EUV) and a corresponding intensity plot showing intensity as a function of distance across the detector. For the image and plot shown in FIG. 7, it can be seen that the angular emission distribution of EUV light is not symmetric and centered. Instead, as best seen in the FIG. 7 plot, in-band EUV intensity is stronger on the left side of the detector. This corresponds an EUV output pulse generated by a laser-droplet interaction in which the laser is off-center relative to the droplet.

FIG. 8 shows an example of a fluorescent converter image (note: light areas indicate in-band EUV) and a corresponding intensity plot showing intensity as a function of distance across the detector. For the image and plot shown in FIG. 8, it can be seen that the angular emission distribution of EUV light is substantially symmetric and centered about the input laser axis. Indeed, as best seen in the FIG. 8 plot, in-band EUV intensity is somewhat balanced on the left and right sides of the detector. This corresponds an EUV output pulse generated by a laser—droplet interaction in which the laser is substantially centered relative to the droplet.

Output signals from the detector 706 may be used to provide a more accurate coupling between the input laser and source material droplet. In particular, output signals from the detector 706 may be used to make adjustments to the focus and steering assembly and/or the focal power adjustment assembly described above or to the target delivery control mechanism 92′ to provide a laser—droplet interaction in which the laser is substantially centered relative to the droplet. This procedure may be performed during light source setup and maintained at constant settings during operational use, or, adjustments to the focus and steering assembly, the focal power adjustment assembly and/or the target delivery control mechanism 92′ may be made during light source operational use (i.e. during utilization of EUV light by a downstream apparatus, e.g. lithography scanner), for example, on a pulse by pulse basis and/or after a so-called “burst” of pulses, when a pulsed laser source 22 (see FIG. 1) is used.

Referring back to FIG. 1, a light source 20 is shown that produces EUV light by plasma formation and generates debris and may include a plasma chamber 26, a laser source 22, which may be a CO₂ gas discharge laser producing a laser beam having a wavelength, λ, e.g. about 10.6 μm, and a laser input window 56 for the plasma chamber. As further shown, the light source may include an etchant source 102, e.g. HBr, HI, Br₂, Cl₂, HCl, H₂ or a combination thereof, for the plasma formation material, e.g. Sn, into the plasma chamber and a heater 104 may be provided for heating deposited plasma formation material on the window 56 to an elevated temperature, t, e.g., a temperature greater than about 150° C., to increase a rate of a chemical reaction between deposited plasma formation material and the etchant. For this environment, a standard CO₂ laser window made of ZnSe may not be chemically stable when exposed to one or more of the etchants described above and/or elevated temperature. As a consequence, the light source 20 may employ a window 56 that is made of a material such as KBr, CsBr or CsI. These materials are substantially transparent to light at a wavelength of about 10.6 μm, and may be chemically stable when exposed to one or more of the above-described etchants at elevated temperatures. For example, KBr can be used with Hbr as the etchant and CsI can be used if HI is the etchant. Other combinations may also be suitable depending on etchant concentration and temperature. Alternatively, the window 56 may be made by coating a material such as ZnSe on the surface exposed to the plasma chamber interior with KBr, CsBr or CsI. For example, a KBr coating can be used with Hbr as the etchant and a CsI coating can be used if HI is the etchant. Other combinations may also be suitable depending on etchant concentration and temperature. Since KBr is highly hydroscopic, caution may need to be taken to avoid exposure to room air for prolonged periods of time. An in-situ heater may be provided when using a KBr chamber window to reduce absorbed moisture. The side of the window 56 that is not exposed to the etchant may be coated with a protection coating, so it is not sensitive to moisture.

Another technique which may be employed to reduce moisture absorption involves in-situ coating a ZnSe window with KBr, CsBr or CsI after the window is installed in the vacuum chamber and the air is pumped out. FIG. 1 shows that a sputter gun 110, e.g. RF magnetron gun, may be disposed inside the chamber 26 to deposit a coating of KBr, CsBr or CsI on the window 56. In one implementation (not shown), the chamber window may be oriented at Brewsters angle relative to the incident laser bean to reduce losses due to reflectivity. For the implementation, Brewsters angle may be calculated with the refractive index of the KBr, CsBr or CsI coating layer that is deposited on the chamber window (for example, the refractive index of KBr is close to 1.52 at 10.6 microns). Alternatively, other methods of in-situ coating a window with a KBr, CsBr or CsI film can be used, such as CVD.

It will be understood by those skilled in the art that the aspects of embodiments of the present invention disclosed above are intended to be preferred embodiments only and not to limit the disclosure of the present invention(s) in any way and particularly not to a specific preferred embodiment alone. Many changes and modification can be made to the disclosed aspects of embodiments of the disclosed invention(s) that will be understood and appreciated by those skilled in the art. The appended claims are intended in scope and meaning to cover not only the disclosed aspects of embodiments of the present invention(s) but also such equivalents and other modifications and changes that would be apparent to those skilled in the art. While the particular aspects of embodiment(s) described and illustrated in this patent application in the detail required to satisfy 35 U.S.C. §112 are fully capable of attaining any above-described purposes for, problems to be solved by or any other reasons for or objects of the aspects of an embodiment(s) above described, it is to be understood by those skilled in the art that it is the presently described aspects of the described embodiment(s) of the present invention are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the present invention. The scope of the presently described and claimed aspects of embodiments fully encompasses other embodiments which may now be or may become obvious to those skilled in the art based on the teachings of the Specification. The scope of the present invention is solely and completely limited by only the appended claims and nothing beyond the recitations of the appended claims. Reference to an element in such claims in the singular is not intended to mean nor shall it mean in interpreting such claim element “one and only one” unless explicitly so stated, but rather “one or more”. All structural and functional equivalents to any of the elements of the above-described aspects of an embodiment(s) that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Any term used in the specification and/or in the claims and expressly given a meaning in the Specification and/or claims in the present application shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term. It is not intended or necessary for a device or method discussed in the Specification as any aspect of an embodiment to address each and every problem sought to be solved by the aspects of embodiments disclosed in this application, for it to be encompassed by the present claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element in the appended claims is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited as a “step” instead of an “act”. 

1. An EUV light source comprising; a laser source; a plasma chamber; and a focusing mirror for focusing a laser beam from said laser source to a focal spot within said plasma chamber.
 2. An EUV light source as recited in claim 1 wherein said focusing mirror is an off-axis parabolic mirror.
 3. An EUV light source as recited in claim 1 wherein said focusing optic is moveable to selectively move the focal spot.
 4. An EUV light source as recited in claim 3 further comprising a target delivery system for delivering a plasma source material to a plasma formation site in the plasma chamber.
 5. An EUV light source as recited in claim 4 wherein said target delivery system comprises a droplet generator.
 6. An EUV light source as recited in claim 4 further comprising a target position detector for generating a signal indicative of target position.
 7. An EUV light source as recited in claim 6 wherein said focusing optic is moveable in response to said signal from said target position detector to locate the focal spot along a target trajectory.
 8. An EUV light source as recited in claim 6 further comprising a steering optic disposed in said plasma chamber for steering the laser beam focal spot in said plasma chamber.
 9. An EUV light source as recited in claim 8 wherein said steering optic and said focusing optic are moveable together in a first direction to move the focal spot along a path parallel to the first direction.
 10. An EUV light source as recited in claim 8 wherein said steering optic is independently moveable in two dimensions.
 11. An EUV light source as recited in claim 8 wherein said steering optic is moveable in response to said signal from said target position detector to locate the focal spot along a target trajectory.
 12. An EUV light source comprising; a laser source; a plasma chamber; a focusing optic for focusing a laser beam from said laser source to a focal spot within said plasma chamber; and an actuator moving said focusing optic to selectively move the focal point within the plasma chamber.
 13. An EUV light source as recited in claim 12 further comprising a target position detector for generating a signal indicative of target position and wherein said focusing optic is moveable in response to said signal from said target position detector to locate the focal spot along a target trajectory.
 14. An EUV light source as recited in claim 12 further comprising a steering optic for steering the laser beam focal spot in said plasma chamber and wherein said steering optic and said focusing optic are moveable together in a first direction to move the focal spot along a path parallel to the first direction.
 15. An EUV light source comprising; a laser source; a plasma chamber; a focusing optic for focusing a laser beam from said laser source to a focal spot within said plasma chamber; and an optical assembly to adjust a focal power of said focal spot.
 16. An EUV light source as recited in claim 15 wherein said optical assembly comprises a z-fold telescope.
 17. An EUV light source as recited in claim 16 wherein said z-fold telescope comprises two spherical mirrors.
 18. An EUV light source as recited in claim 15 wherein said focusing optic is a mirror.
 19. An EUV light source as recited in claim 15 further comprising a conical shroud positioned in said chamber to at least partially shield said focusing optic from debris generated in said plasma chamber.
 20. An EUV light source as recited in claim 19 further comprising a system for flowing a gaseous etchant in said shroud.
 21. An LPP EUV light source comprising: a laser source generating a laser beam; a source material droplet generator to generate droplets for exposure to said laser beam and produce an EUV emission; a detector for measuring an angular EUV emission distribution and outputting a signal indicative thereof; and a control system responsive to said signal to increase a coupling between said laser beam and said droplets. 